The present invention relates to a circulating fluidized bed steam generator and a method for operating the circulating fluidized bed steam generator for producing a carbon dioxide end product.
U.S. Pat. No. 5,175,995 to Pak et al describes a conventional power generation plant operable to burn fuel with air in a combustor so as to thereby provide combustion gas energy to drive a steam or gas turbine. In those versions of such conventional power generation plants which combust natural gas, petroleum fuel gas or coal gas as the fuel for the combustion process, this fuel includes carbon components such as carbon (C), carbon monoxide (CO), and other hydrocarbons (CmHn). Accordingly, the flue gas produced by a combustion process which combusts the fuel in the presence of air is comprised of carbon dioxide (CO2), nitrogen oxide (NOx), and sulfur oxide (SOx) as well as nitrogen gas (N2).
The ""995 Pak et al patent further notes that the release of gases such as NOx, SOx, and CO2 into the atmosphere creates environmental pollution. Conventional power generation plants have conventionally countered such pollution by the deployment of removal equipment such as scrubbers to remove the NOx and SOx pollutants. Moreover, removal equipment has been deployed to remove the carbon dioxide (CO2) present in the flue gas including removal equipment of the type which utilizes a sorbent to selectively absorb the carbon dioxide (CO2) from the flue gas. However, this solvent approach, according to the ""995 Pak et al patent, disadvantageously requires additional heat energy to heat the solvent and it is not practical to provide the relatively long contact time between the solvent and the carbon dioxide (CO2) for the solvent to fully absorb the carbon dioxide (CO2).
The ""995 Pak et al patent discloses several versions of a closed combined cycle type power generation plant which purports to ameliorate some of the disadvantages of the carbon dioxide (CO2) absorbing solvent approach. In each version of the power generation plant disclosed in this reference, fuel is supplied to a combustor in the presence of oxygen instead of in the presence of air so as to produce a combustion gas (flue gas) which mainly includes a water component and carbon dioxide (CO2). This combustion gas is handled so as to separate the water component and the carbon dioxide (CO2) with the separated carbon dioxide (CO2) being recycled as the working fluid for driving a turbine of the combined cycle power generation plant. Since the combustion of the fuel in presence of oxygen instead of air substantially eliminates the creation of NOx and, further, since the carbon dioxide (CO2) is retained within the closed cycle as a working fluid, the approach disclosed by the ""995 Pak et al patent advantageously avoids the discharge of NOx as well as carbon dioxide (CO2).
U.S. Pat. No. 4,498,289 to Osgersby also discloses a power system which combusts a hydrocarbonic fuel in the presence of oxygen in lieu of air so as to obtain a working fluid comprised of carbon dioxide (CO2). While the ""995 Pak et al patent and the ""289 Osgersby patent each disclose an arrangement for reducing carbon dioxide (CO2) emissions via the combustion of fuel in the presence of oxygen instead of air, the art could still benefit from an arrangement for new or existing power generation system designs which offers the flexibility to both to produce carbon dioxide (CO2) as a desirable end product and to use carbon dioxide (CO2) as support to the combustion process. Also, the art could benefit from an arrangement comprising a circulating fluidized bed steam generator whose combustion temperature can be controlled with the aid of up to no more than one-half the typical requirement for flue gas recirculation required by a comparably performing pulverized coal steam generator.
It is an object of the present invention to provide an arrangement for new or existing power generation system designs which offers the flexibility both to produce carbon dioxide (CO2) as a desirable end product and as support to the combustion process.
It is another object of the present invention to provide an arrangement for producing liquid carbon dioxide (CO2) which improves the heat output of a fossil fuel fired power generating system.
According to one aspect of the present invention, a method for operating a circulating fluidized bed steam generator is provided for new or existing power generation system designs which offers the flexibility to use carbon dioxide (CO2) both as a desirable end product and as support to the combustion process. The method includes the step of introducing a substantially pure oxygen feed stream into the circulating fluidized bed steam generator and the step of combusting a fuel in the presence of the substantially pure oxygen feed stream to produce a flue gas having carbon dioxide and water vapor as its two largest constituent elements by volume.
The method for operating a circulating fluidized bed steam generator in accordance with the present invention includes the steps of introducing a substantially pure oxygen feed stream into the circulating fluidized bed steam generator, combusting a fuel in the presence of the substantially pure oxygen feed stream to produce a flue gas having carbon dioxide and water vapor as its two largest constituent elements by volume, passing the flue gas through an oxygen feed stream pre-heater at which heat from the flue gas is transferred to the oxygen feed stream, separating the flue gas into an end product portion and a recycling portion, and directing the recycling portion of the flue gas to the circulating fluidized bed steam generator to contribute to the combustion process therein. Preferably, the method also includes cooling and compressing the end product portion of the flue gas so as to yield carbon dioxide in a liquid phase.
According to another aspect of the present invention, there is provided a circulating fluidized bed steam generator which includes means for introducing a substantially pure oxygen feed stream into the circulating fluidized bed steam generator, means for combusting a fuel in the presence of the substantially pure oxygen feed stream to produce a flue gas having carbon dioxide and water vapor as its two largest constituent elements by volume, means for passing the flue gas through an oxygen feed stream pre-heater at which heat from the flue gas is transferred to the oxygen feed stream, means for separating the flue gas into an end product portion and a recycling portion, and means for directing the recycling portion of the flue gas to the circulating fluidized bed steam generator to contribute to the combustion process therein. Preferably, the circulating fluidized bed steam generator also includes means for cooling and compressing the end product portion of the flue gas so as to yield carbon dioxide in a liquid phase.